Making superphosphate



Patented Feb. 16, 1937 UNITED STATES PATENT OFFICE MAKING SUPERPHOSPHATEa corporation No Drawing. Application October 9, 1935, Serial No. 44,272

3 Claims.

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) This application is made underthe act of. March 3', 1883', as amended by the act of" April 30, 1928,and the invention herein described, if patented, may be manufactured andused by or 5 for the Government for governmental. purposes without thepayment to me of any royalty thereon.

This invention relates to a process of making triple superphosphate,particularly from fine phosphate rock and concentrated phosphoric acid;

One of the objects of my invention is to carry out the several steps ofthe process of making superphosphate from fine phosphate rock; orcalcium carbonate, and concentrated phosphoric acid with a expenditureof power. Another object oi'my' invention is to reduce to a minimum thetime required to bring the superphosphate to a granular, essentiallyd'ry condition such that it may be stored without subsequentagglomeration.

When fine phosphate rock, or fi'ne calcium oarbonate, and concentratedphosphoric acid are mixed quickly, there is a short period or timeduring which the mixture remains fluid. This period of time is short,but it is not too short to permit a thorough mixing of the fine rock andacid if a sufliciently high speed mixer be used. As soon as the chemicalreaction between the fine phosphate rock, or the fine calcium carbonate,and the concentrated phosphoric acid begins, the mixture starts tothicken. The chemical reaction also liberates heat which causes a rapidincrease in the rate of reaction. The result is that the mixturethickens very rapidly and in a short time reaches a. condition in whichit may be crumbled into small piecesvery easily. Subsequent to thisstage the material hardens and sets up into reasonably hard, discretemasses.

In my co-pending application, application Ser. No. 726,757, filed May21, 1934, now Patent 2,053,266 I.have disclosed the method of makingsuperphosphate from fine phosphate rock and concentrated phosphoric acidwhich involves the intimate mixing of the mixture of phosphate rock andacid through the stage of high fluidity and this mixing continuedthroughout the plastic stage until the mixture breaks up into smalldisphosphate which requires a minimum expenditure of power and reducesto a minimum the time required to bring the superphosphate to agranular, essentially dry condition, which comprises the combination ofprocess steps of efiectively 5 mixing the charge in a paddle type mixeroperated at a sufliciently high speed to insure a thorough mixing of thecharge while in a fluid condition and only for the duration of the highfluidity of the mixture, conveying away the product without furthermixing for a sufiicient period of time. for the reaction to continue,during which themixture passes through the plastic stage and crumbles ondisturbing the mixture, such as by discharging the contents of the firstconveyor onto a second conveyor, and conveying the crumbly mixture for asufficient time for the reaction to be continued further so that themixtureforms lumps which do not agglomerate in storage.

One example of the operation of my process is given for making themono-calcium phosphate from fine phosphate. rock and. concentratedphosphoric. acid. A phosphate rock, ground so that 20% passes a 200 meshscreen, and concentrated phosphoric acid, containing 75% by weight ofH3PO4, were charged at the rate of 900 lbs. per hour and 1,040 lbs. perhour respectively into a continuous vertical paddle or armmixer with ahorizontal section of approximately 4 x6 and with a depth ofapproximately 4" equipped with two vertical shafts, each 1 in diameter,carrying blades which describe a circle of approximately 4" in diameter.The material was mixed rapidly While in the fluid state and for theperiod of duration of high. fluidity of the mixture and then dischargedfrom the mixer. The time required for the passage of the charge throughthe mixer was approximately 15 seconds. The power consumption was at therate of approximately 4 H. P., which is equivalent to 0.28'H. P.-hr. perton. The mixture was discharged onto a conveyor and moved along withoutsubstantial disturbance for a sufficient period of time for the reactionto continue while the mass passed through the plastic stage and set upsuificiently so that it crumbled on disturbance. This time wasapproximately two to three minutes. The contents of this first conveyorwere discharged onto a second conveyor and in so doing the mass broke upinto crumbly lumps. This crumbly material was conveyed away on thesecond conveyor for a suflicient period of time for the reaction tocontinue further and the lumps to reach a condition in which they didnot agglomerate in storage. This time was approximately ten minutes. Thecontents of the second conveyor were discharged onto the storage pilewhere the product was available for a minimum glomerates.

of processing at the time of bagging and shipping.

The small amount of power for the mixing, as given above, may becompared by way of example with the results obtained in a certaincommercial installation in which a continuous horizontal mixer wasdesigned not only to accomplish the mixing through the stage of highfluidity but through the plastic stage until the product was dischargedin the form of individual pieces or agglomerates. In this commercialunit the phosphate rock, ground so that 80% passed through a 200 meshscreen, and phosphoric acid, containing 76% by weight of I-I3P04, werecharged at thevrate of 2600 lbs. per hour and 2900lbs. per hour re--spectively. The continuous horizontal mixer was approximately 10'x32",with a depth of 24", and equipped with two horizontal shafts, eachapproximately 5" in diameter and carrying blades which described acircle of approximately 17'' vin diameter. The material --was mixedthrough the fluid and plastic stages and was discharged in the form ofindividual pieces or ag- The time required for the passage of the chargethrough the mixer was approximately two minutes. -The power consumptionwas at the rate of approximately H. P., which is equivalent to 11 H.P.-hr. per ton of charge.

Attention is calledto the factv that in carrying out the mixing steps inthe continuous horizontal mixer mentioned above, the "power required wasllH. P.-hr. per ton of charge, while in carrying out the mixing step inaccordance with the present invention, the power required was 0.28 H.P.- hr. per ton.

Itis'evident that there are numerous factors 7 which will influenceconditions for. the most satisfactory operation of my invention, theactual limits of which cannot'be established except by detailed study ofeach set of raw materials in the intermediate and finished productinvolved.

'Fine phosphate rock or a calcium carbonate,

such as fine limestone or fine marble, may be used as the source of thecalcium compound which may be reacted with the concentrated phosphoricacid to form the superphosphate, which is essentiallymono-calciumphosphate. The calcium compound used should be'reduced to arelatively fine state of division but the re-' 'quirements in thisconnection will vary, depending upon the nature of this constituent ofthe charge and Whether or not it is a mineral or an intermediate or arefined product. In using the phosphate rock with the concentratedphosphoric acid, it has been found desirable to have a substantialproportion of the rock of such a size that it will pass through a 100mesh screen.

The concentratedphosphoric acid used should contain at least 657% byweight of H3PO4, with concentrations of 75% to 85% normally beingpreferred. A'small proportion of concentrated sulfuric acid in theconcentrated phosphoric. acid another way, the high speed mixer formixing mixer is conveyed away at a slow rate of speed for a sufiicientlength of time to permit the chemical reaction to proceed until thematerial has passed through the plastic stage and will crumble ondisturbance. After any reasonable disturbance, such as moving from thefirst conveyor to a second conveyor, the material is moved along at aslow rate of speed for a sufficient time for the reaction to haveproceeded to such an extent that the crumbly material sets into lumpswhich do not agglomerate in storage. This material on the'secondconveyor is ordinarily discharged onto a storage pile from which it iswithdrawn for sizing and packing for shipment. that the time requiredfor the steps of mixing, conveying, disintegrating, and furtherconveying the mixture, usually 10 to 20 minutes, is of such an order asto be usedpractically in'manufactur- It will be seen ing and storagebuildingsof the type commonly used in the fertilizer industry. v It willbe seen, therefore, that this invention 1 actually may be carried out bythe modification departing from its of certain details without spirit orscope.

I claim: 7 V e r I 1. Process of treating phosphate rock withconcentrated phosphoric acid, which comprises,

mixing the fine phosphate rock and the concentrated phosphoric acid,containing 75% to by weight of H3PO4, in the proportions to formmono-calcium phosphate, in a paddle type mixer. op'eratedat asufilciently high speed to insure a thorough mixing .of the charge whilein a highly fluid condition and for the duration of the high fluidity ofthe mixture, which is approximately one minute conveying away themixture without ;further agitation during this first conveying periodfor a sufficient period of time for the reaction to continue, for themixture to pass through the plastic stage, and for the mixture tocrumble on disturbance, which is a period of approxij mately two tothree minutes; discharging the contents of the conveyor onto a secondconveyor in such a manner that the material is broken up and crumbledthereby; conveying away the crumbly mixture without further agitationduring this second conveyingjperiod, for a sufficient period of time forthe reaction to continue further and for the mixture to form lumps whichdo not agglomerate in storage, which is approxithe finelimestone and theconcentrated phosphoric acid, containing 75% to 85% byweight of H3PO4,in the proportions to form mono-calcium phosphate, in a paddle typemixer operated at a sufficiently high speed to insure a thorough mixingof the charge while in a highly fiuid condi- ,60 mately eight to twelveminutes; and discharging tion and for the duration of the high fluidityof the mixture; conveying away the mixture without further agitationduring this first conveying period for a sufiicient period of time forthe reaction to continue, for the mixture to pass through :15

the plastic stage, and for the mixture to crumble on disturbance;discharging the contents of the conveyor onto a second conveyor in sucha manner that the material is broken up and crumbled thereby; conveyingaway the crumbly mixture without further agitation during this secondconveying period for a sufiicient period of time for the reaction tocontinue further and for the mixture to form lumps which do notagglomerate in storage; and discharging the contents of the secondconveyor.

3. Process of treating a calcium compound, selected from the groupconsisting of phosphate rock and calcium carbonate, with concentratedphosphoric acid, which comprises, mixing the fine calcium compound andthe concentrated phosphoric acid, containing at least 65% by weight ofH3PO4, in the proportions to form mono-calcium phosphate, in a paddletype mixer operated at a sufficiently high speed to insure thoroughmixing of the charge while in a highly fluid condition and for theduration of the high fluidity of the mixture; conveying away the mixturewithout further agitation during this first conveying period for asufficient period of time for the reaction to continue, for the mixtureto pass through the plastic stage, and for the mixture to crumble ondisturbance; discharging the contents of the conveyor onto a secondconveyor in such a manner that the material is broken up and crumbledthereby; and conveying away the crumbly mixture without furtheragitation during this second conveying period for a sufiicient period oftime for the reaction to continue further and for the mixture to formlumps which do not agglomerate in storage.

HARRY A. CURTIS.

